Vol. 5 , No. 6 , November-December 1972
HELIX-COIL STABILITY CONSTANTS FOR AMINOACIDSIN WATER 739
chroism spectra in Figure 3 are both characteristic of randomcoil polypeptides. A possible explanation for the quantitative differences in the circular dichroism spectra may reside in the different interaction of the polypeptides with the solvent. The expansion coefficient for poly(L-glutamic acid) a t 37" in 0.3 M sodium phosphate, p H 7.85, is 1.34 (=k1.4Z),lg which is decidedly greater than the result in Table I1 for poly(N5-whydroxyethyl-L-glutamine) in water a t 30". The spectra in Figures 3 and 4 could be explained in a qualitative manner if the greater expansion of poly(L-glutamic acid) is accompanied
by more heavily weighting values of 4 and $ which lead to positive circular dichroism near 215 nm and intense negative circular dichroism near 200 nm. The characteristic ratios of poly(y-benzyl L-glutamate), poly(i3-benzyl L-aspartate), and poly(L-lysine) were also determined to be close to nine by Brant and Flory.lg Conclusions regarding their circular dichroism under the conditions used are not possible because the solvents (dichloroacetic acid, m-cresol, and 1 M sodium bromide) d o not transmit light in the relevant spectral range.
Helix-Coil S'tability Constants for the Naturally Occurring Amino Acids in Water. VI. Leucine Parameters from Random Poly ( hydroxypropylglutamine-co-L-leucine ) and ) Poly ( hydroxybutylglutamine-co-L-leucine J. E. Alter,2 G. T. Taylor, and H. A. Scheraga*
Department of Chemistry, Cornell University, Ithaca, New York 14850. ReceivedJune 12,1972
ABSTRACT: The synthesis and characterization of water-soluble random copolymers containing L-leucine with either N j 4 3 hydroxypropyl)-L-glutamine or N6-(4-hydroxybutyl)-~-glutamineare described, and the thermally induced helix-coil transitions of these copolymers in water have been studied. The incorporation of L-leucine was found to increase the helix content of the polymer. The Zimm-Bragg parameters u and s for the helix-coil transition in poly(L-leucine)in water were deduced from an analysis of the melting curves of the copolymers in the manner described in earlier papers. Within the limits of error of both methods, the values of u and s for poly(L-leucine) were found to be in good agreement with those obtained earlier using sandwich-type block.copolymers of D,L-lysine and L-leucine in water, thus giving additional weight to the view that the conformational state of an amino acid residue in a polypeptide or protein, in first approximation, is essentially independent of the chemical nature of its neighbors.
T
he use of the "host-guest" technique for the evaluation of the helix-coil stability constants of various amino acids in water has been illustrated in earlier papers of this I n this method, a water-soluble, a-helical host homopolymer is selected, and various amounts of a guest residue are incorporated into it to form random copolymers. By examining the influence of the guest residues o n the helix-coil transition of the host homopolymer, it is p ~ s s i b l e ~to- ~determine the Zimm-Braggs parameters u and s for the guest residues. Two host homopolymers, poly[N5-(4-hydroxybutyl)-~-glutamine] (PHBG) and poly[Nj-(3-hydroxypropyl)-~-glutamine] (PHPG) have been used in this The first is quite helical a t room temperature, and the second much less so. The choice of host is determined primarily by the anticipated (1) This work was supported by research grants from the National Science Foundation (No. GB-28469x1) and from the National Institute of Arthritis and Metabolic Diseases, U. S. Public Health Service (NO.AM-08465). (2) N I H Predoctoral Trainee, 1969-1972. (3) P. H. Von Dreele, D. Poland, and H. A. Scheraga, Mucromolecules, 4,396 (1971) (hereinafter called paper I). (4) P. H . Von Dreele, N. Lotan, V. S. Ananthanarayanan, R. H. Andretta, D. Poland, and H . A. Scheraga, ibid., 4, 408 (1971) (hereinafter called paper 11). (5) V. S. Ananthanarayanan, R. H . Andreatta, D . Poland, and H. A. Scheraga, ibid., 4, 417 (1971) (hereinafter called paper 111). (6) I
